Electronic module and method for producing an electric functional layer on a substrate by blowing powder particles of an electrically conductive material

CROSS REFERENCE TO RELATED APPLICATIONS

0001This application is based on and hereby claims priority to German Application No. 10 2006 037 532.7 filed on Aug. 10, 2006 and PCT Application No. PCT/EP2007/057883 filed on Jul. 31, 2007, the contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

0002The invention relates to a method for producing an electric functional layer on a surface of a substrate, on which at least one component, in particular a semiconductor chip, is arranged.

0003The starting point for the production of an electronic module is a semi-finished product with a substrate to which a structured metal layer with metal or contact surfaces is applied. Some of the contact surfaces feature one or several components, for example a semiconductor chip or a passive component. The component or components is or are connected to the respective contact surface by an adhesive agent, usually solder. If one of the components exhibits a rear-side contact, i.e. a contact facing towards the substrate, the adhesive agent creates not only a mechanical and but also an electrical connection to the respective contact surface.

0004For electrical contacting some of the components exhibit a number of contact surfaces on their top side facing away from the substrate. The electrical connection between the contact surfaces themselves and/or one of the contact surfaces of the metal layer is usually formed by using bond wires.

0005Alternatively, the electrical connections between the contact surfaces of the components and/or a contact surface of the metal layer can be produced by planar connection technology, in which a surface of the semi-finished product is initially covered with an insulation layer, for example a plastic foil of an insulating material. At the locations of the contact surfaces openings are inserted in the insulation layer so that the contact surfaces are not covered. Subsequently a thin metal layer is applied by sputtering, vapor deposition or other methods to produce thin contact layers over the entire surface of the insulation layer and the inserted openings. The sputter layer is for example an approx. 50-nm layer of titanium and an approx 1-μm layer of copper. Onto this sputter layer a light-sensitive foil (photo foil), usually an insulating material, is applied. The thickness of the photo foil is between 20 and 200 μm and in a further step it is exposed and developed according to the desired conductive structure.

0006Exposure usually takes place using a mask with which the layout of the conductive structure is transferred to the photo foil. The sections of the photo foil intended to form the later electrically conductive structure or functional layer are shaded by the mask. The non-exposed sections of the photo foil can be removed in a further step to reveal the sputter layer, or more precisely the copper surface, underneath. By immersing the prepared semi-finished product in an electrolyte bath, in particular a copper electrolyte bath, a copper layer with a thickness of approx. 20 to 200 μm is produced by galvanic growth. In a next step, referred to as stripping the photo foil, the photo foil still on the surface is removed in the places where no electrically conductive structure is required. In a final step, referred to as differential etching, the sputter layer of titanium and copper is removed from the entire surface so that only the desired conductive structure or functional layer remains.

0007If the components are configured as power semiconductor components the conductive structure or functional layer is usually made of copper. The required layer thickness ranges from 20 to 500 μm.

0008Planar connection technology has the advantage that the electronic module produced is much lower in height than electronic modules which have conventional wires. Owing to the numerous steps required, however, this connection technology entails higher costs.

SUMMARY

0009It is therefore one potential object to specify a method for producing an electric functional layer on a surface of a substrate which furnishes reliable electrical contacting and at the same time is favorable in terms of cost.

0010The inventor propose producing an electric functional layer on a surface of a substrate, on which at least one electronic component, in particular a semiconductor chip, is arranged, the electric functional layer is formed by selectively blowing powder particles of an electrically conductive material onto the surface of the substrate so that on impact with the substrate they form a dense and strongly adhering layer.

0011The proposed has the advantage that an electrical connection can be produced cost-effectively and quickly by planar technology. The electronic module produced by the method only has a low installation height. Compared with thermal flame spraying, highly dense layers can be produced. The functional layer exhibits high electrical as well as high thermal conductivity. The functional layer is wear resistant and very hard, even though only a low heat input into the substrate takes place during production. As very high application rates exceeding 100 μm per minute are possible the process times for the manufacture of electric functional layers can be reduced to minutes.

0012To apply the particles, which are in powder form, the particles are injected into a process gas which has been heated up and accelerated to supersonic speed, preferably to a speed range between 300 and 1,200 m/s. The process gas is accelerated to supersonic speed preferably by expansion of the process gas in a nozzle.

0013The particles injected into the process gas are preferably made of metal, in particular copper, titanium, silver, nickel and mixtures thereof. It is particularly preferable for the particles to be injected into the process gas as nanoparticles.

0014The electric functional layer is produced according to the principle of cold gas spraying. Cold gas spraying is a coating method in which the coating material is applied to the substrate in powder form at very high speed. For this purpose, a process gas heated up to a few 100° C. is accelerated to supersonic speed by expansion in a Laval nozzle. The powder particles are then injected into the gas jet. The injected particles are accelerated to such a high speed that without any prior melting they form a dense and strongly adhering layer on impact with the substrate. The kinetic energy at the time of impact as a rule is not enough to cause the particles to melt completely.

0015According to one embodiment, the electric functional layer is applied to the entire surface of the substrate. This makes it possible to produce all-over or only partial shields to improve EMC behavior.

0016The electric functional layer can also be applied selectively to the surface of the substrate in order to create a conductor path structure.

0017To locally improve heat removal, provision can also be made during the manufacture of the electric functional layer for producing sections of different thickness. The thicker the conductive layer, the better the heat removal.

0018The proposed method enables passive components to be formed on the substrate surface during production of the electric functional layer. These could for example be resistors in meandering structure or capacitors (charge-storage diodes). It is also conceivable for three-dimensional structures with a differing component topography to be produced.

0019The functional layer can be formed from a number of layers of different or the same materials.

0020In principle any layer thicknesses can be produced in the electric functional layer. The thickness of the functional layer preferably lies between 20 μm and 5 mm.

0021Expediently, a sacrificial layer is applied to the surface of the substrate before application of the electric functional layer. The sacrificial layer prevents the components on the substrate from being damaged by the particles impacting at high kinetic energy.

0022To produce the sacrificial layer a thin metal layer is formed by sputtering, vapor deposition or PVD processes to produce thin contact layers. If the thickness of the sputter layer (usually in the region of 1 μm) is not enough to absorb the kinetic energy, provision can be made to increase the thickness of the sputter layer by galvanic deposition. Expediently, the sacrificial layer then produced exhibits a thickness of 1 μm to 50 μm.

0023The particles are preferably applied to the surface of the substrate by using a structured mask. The material used for the mask can be a highly elastic material which absorbs the kinetic energy of the particles on impact. Alternatively, the material used for the mask can be a soft material which changes shape when the particles impact. The material used for the mask can be a polymer for example which results in poor adhesion of the particles on the mask. A porous or brittle material can also be used as the material for the mask which breaks off during cold gas spraying. Such materials are for example made using ceramic pastes. A preformed metal mask with a surface coating can also be used as the mask, whereby the surface coating prevents adhesion of the particles on the surface. The surface coating forms a “non-stick coating”. Finally, a structured photo foil with a silicone surface can be used as the mask.

0024Expediently, the surface of the substrate with the at least one component is provided with an insulation layer to which the electric functional layer is applied. In one configuration the insulation layer is made of organic or inorganic insulation material, in particular plastic, glass or ceramic.

0025The inventors also propose an electronic module including a substrate, on which at least one electronic component, in particular a semiconductor chip, is arranged and an electrically conductive functional layer. The electrically conductive functional layer is produced according to the described method.

BRIEF DESCRIPTION OF THE DRAWINGS

0026These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which:

0027<figref idref="DRAWINGS">FIG. 1</figref> shows a schematic representation of a section through an electronic module produced using the method in accordance with the invention;

0028<figref idref="DRAWINGS">FIG. 2</figref> is a side view of an electric functional layer formed in sections; and

0029<figref idref="DRAWINGS">FIG. 3</figref> is a side view of a layered electric functional layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

0030Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawing, wherein like reference numerals refer to like elements throughout.

0031A metal layer <b>11</b>, for example made of copper, is formed on a top side of a substrate <b>10</b>. The substrate <b>10</b> can for example be made of ceramic, printed circuit board material or flexible tape. The metal layer <b>11</b> exhibits a number of contact surfaces <b>13</b>, <b>14</b>, although only two contact surfaces are represented in the exemplary embodiment. On a bottom side opposite the top side of the substrate <b>10</b> a metal layer <b>12</b> is likewise formed, on which for example a heat sink can be arranged. For this purpose the metal layer <b>12</b> is, for example, formed over the entire surface and also made of copper.

0032For example a solder paste, a component <b>15</b>, for example a power semiconductor component, is fixed to the contact surface <b>14</b> by an adhesive agent <b>16</b>. The component exhibits a contact surface <b>22</b> on its top side facing away from the substrate. The surface of the described arrangement (hereinafter also referred to as the semi-finished product) is covered with an insulation layer <b>17</b>. The insulation layer <b>17</b> may have been applied by a vacuum process. The thickness of the insulation layer <b>17</b> depends on the use of the finished electronic module, in particular on the voltages arising during operation of the module. Typically, the thickness of the insulation layer <b>17</b> ranges between 100 and 400 μm. The insulation layer <b>17</b> is applied in such a way that between the insulation layer <b>17</b> and the surface of the semi-finished product, comprising the metalized substrate with the component attached thereto, no air inclusions exist which would entail the risk of voltage flashovers. When applying the insulation coating <b>17</b>, a method is preferably used which exerts no or hardly any stretching or compressive force on the side edges of the component. The insulation layer <b>17</b> therefore preferably exhibits a constant thickness over the entire surface of the semi-finished product, as a result of which the electrical properties of the finished electronic module are optimized. In the area of the contact surfaces <b>13</b> and <b>22</b> openings are inserted in the insulation layer <b>17</b>. The openings are usually inserted by a laser.

0033After the insulation layer <b>17</b> has been applied and the openings have been inserted the surface of the semi-finished product which is to be covered with an electric functional layer is provided with a mask <b>20</b>. The mask <b>20</b> can, as shown in the exemplary embodiment, be applied directly onto the surface of the semi-finished product to be provided with the functional layer. The mask can however also be arranged at a distance from the surface in order to create the desired shading.

0034In the exemplary embodiment the mask exhibits merely by way of example a single, large opening <b>21</b> within which the contact surface <b>22</b> of the component <b>15</b> and the contact surface <b>13</b> of the metal layer <b>11</b> are uncovered. In principle the mask is designed in such a way that areas which are not intended to be covered with an electric functional layer are shaded or concealed by the mask.

0035In the area of the opening <b>21</b> of the mask <b>20</b> a sacrificial layer <b>18</b> is applied to the surface of the semi-finished product. In the exemplary embodiment the sacrificial layer <b>18</b> was applied after application of the mask <b>20</b>, which is not, however, imperative. The sacrificial layer <b>18</b> is made of a conductive material and is applied directly to the uncovered contact surfaces <b>13</b> and <b>22</b>, where the insulation layer <b>17</b> exhibits openings. Otherwise, the sacrificial layer <b>18</b> runs on the insulation layer <b>17</b>. The sacrificial layer <b>18</b> therefore adapts to the three-dimensional surface structure of the semi-finished product.

0036The sacrificial layer <b>18</b> can be produced by sputtering. If necessary, the sputter layer can be made thicker by galvanic deposition. Preferably the sacrificial layer exhibits a thickness of 1 to 10 μm. The sacrificial layer <b>18</b> ensures that in particular the component <b>15</b> is not damaged when the electrically conductive particles are subsequently applied at high kinetic energy.

0037The powder particles for example of copper, titanium, silver, nickel or mixtures thereof, preferably in the form of nanoparticles, are applied by being blown onto the surface of the semi-finished product fitted with the mask <b>20</b>. For this purpose, the particles are injected into a process gas which has been heated up and accelerated to supersonic speed. The process gas is accelerated for example by expansion through a nozzle. The process gas preferably exhibits a speed between 300 and 1,200 m/s. Mainly inert gases, in particular nitrogen, can be considered for use as carrier gases. The method for applying particles in powder form is also referred to as cold gas spraying.

0038Gradient materials which exhibit a metal as contact surfaces as well as a barrier layer of titanium can be applied.

0039The thickness of the electric functional layer <b>19</b> is determined by the length of time for which the particles are blown onto the surface of the semi-finished product. Thicknesses of more than 100 μm can be achieved per minute. Referring to <figref idref="DRAWINGS">FIG. 2</figref>, the electric functional layer <b>19</b> may be formed in sections <b>19</b>A, <b>19</b>B having different respective thicknesses.

0040The application of the electric functional layer <b>19</b> can take place in several steps in order to obtain a layered structure <b>19</b>C <b>19</b>D of the functional layer <b>19</b>. See <figref idref="DRAWINGS">FIG. 3</figref>. Such a structure is advantageous in order to avoid copper migration. A further advantage exists in that structuring is possible by laser.

0041After the electric functional layer <b>19</b> has been produced the mask <b>20</b> can be removed from the surface of the produced electronic module.

0042Suitable materials for the mask include highly elastic materials, soft materials, porous or brittle materials. The use of a preformed metal mask with a surface coating on which metal does not adhere or a structured photo foil with a silicone surface is also conceivable.

0043Silicone rubber for example can be used as a highly elastic material for a mask. It absorbs the kinetic energy from the particles as they impact on the mask. Soft mask materials change their shape as the particles are applied and therefore lead to poor adhesion between the mask and the particles. As a result, the material not reaching the surface of the semi-finished product can be re-used. Porous or brittle materials, for example ceramic pastes, break off during cold gas spraying and absorb the energy of the particles.

0044In a further embodiment not shown in the FIGURE one or several further metal layers can be applied to the functional layer <b>19</b>, for example exclusively in the vicinity of the component <b>15</b>, to improve the thermal properties. An insulating layer or a cooling element or layer can then in turn be applied.

0045The method proposed by the inventors makes it possible to produce electronic modules with multi-layer or stack structures in which insulation layers and electric functional layers can be arranged alternately on top of each other.

0046An electronic module produced using the proposed method is advantageous in that the electrical connections are realized at a planar level so that the component is low in height. The method in particular not only enables conductor path structures to be produced but also makes it possible to form an electrically conductive layer on extensive surface areas. As a result, the EMC behavior of the electronic module can be improved.

0047The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004).